Diagnostic imaging of early stage breast cancer is essential for decreasing the death rate caused by cancer in the United States. Near-infrared (NIR) optical imaging using external fluorescence contrast agents is an emerging non-invasive modality that can become an important tool in the diagnosis of early-stage breast cancer and prognosis of the disease. To date, most of the work in fluorescence-enhanced optical tomographic imaging is carried out using: (i) small volume phantom or in-vivo animal models that lack clinically relevant depth information;(ii) non-flexible optical probes that are restricted to image only specific tissue volumes or shapes;or (iii) measurement geometries that interrogate limited tissue volumes. In the current application, we propose to develop a hand-held optical fiber probe (~ 2" diameter) to perform fluorescence imaging of breast cancer using large tissue phantoms and in-vitro tissue models. The hand- held optical probe will be unique in terms of its: (i) flexibility to image any tissue shape and volume;(ii) non- compressibility, portability, and patient comfort due to its hand-held based design;and (iii) novel measurement geometry that can interrogate greater tissue volumes with reduced imaging time. The developmental aims are to: (i) design and construction of a hand-held optical with optimal fiber configuration for simultaneous NIR light illumination and collection measurement geometry;(ii) develop and optimize a rapid data acquiring frequency-domain intensified charge coupled device (ICCD) detection system;(iii) perform fluorescence-enhanced absorption imaging studies using tissue-mimicking phantoms and in-vitro tissue samples and assess the precision and accuracy of the fluorescence measurements;and (iv) integrate an undergraduate student with current research at various stages of the proposed work. The milestone of the proposed work is to develop a hand-held optical-based imagerthat is robust, rapid, flexible, non- compressible, comfortable, portable, and applicable for any given breast volume and shape, that is expected to render high-resolution diagnostic and prognostic breast cancer imaging in the clinic.